Frederick, a 38-year-old naturalized citizen from the Caribbean, choked up as she recounted an officer’s demanding to see her paperwork during a routine traffic stop. Ayala also began to cry, recalling how she had to teach her son, who is black, how to interact safely with police.

They hugged.

“It’s nice to have someone who shares our background so they understand people who they are speaking for,” said Frederick, who is upset by President Trump’s handling of immigration.

Across the country, strategists are watching to see whether Democrats can convert voter disapproval with Trump into victory at the ballot box. The first clues may come in November in Prince William County, just the kind of fast-growing, suburban swing district prized by candidates from both parties.

In recent election cycles, Prince William has shifted from mostly white, rural and reliably Republican to diverse, developed and swingy — an example of a changing Virginia. The county voted for George W. Bush twice, then swung to Barack Obama, voting for him twice, and Hillary Clinton last year. It voted for Republican Robert F. McDonnell for governor in 2009, then swung to Terry McAuliffe four years later.

This election, a diverse Democratic slate is hoping to take advantage of those demographic and political shifts to challenge longtime, white male GOP incumbents in state legislative districts carried by Hillary Clinton last November.